Borate compounds (i.e. boron compounds in which boron is bonded only to oxygen) are useful in many industrial applications including the manufacture of glass and ceramics, fire retardancy, wood preservation and corrosion control. For fire retardant use, desirable attributes of borate compounds include ease of manufacture, a relatively high B2O3 content, and a high dehydration on-set temperature. The high dehydration temperature is important because it allows for processing in polymer systems at elevated temperatures. Borates are commonly used as flame retardant additives for polymers. For example zinc borate is often used as a partial substitute for antimony oxide in halogenated flame retard polymer compositions to reduce burn time times and/or afterglow in the standard UL-94 fire retardant test. Ceramics include both the common vitreous types and advanced ceramic materials.
Amine borate compositions are useful in a variety of applications including fertilizing plants (U.S. Pat. No. 4,844,725), inhibiting the corrosion of metals (U.S. Pat. No. 5,100,583), controlling insects and fungi, preserving wood (U.S. Pat. No. 5,061,698), and as boron nitride ceramic precursors (JP 95,102,965). In the case of borates used as precursors to advanced ceramics, important attributes include ease of preparation and an appropriate elemental ratio in the thermal decomposition products. The ability to produce such borate compounds under mild condition (at or near atmospheric pressure in aqueous media) greatly simplifies the manufacturing process, leading to better economics. Also, processes resulting in the production of little or no effluent are generally more economical and environmentally sound.
Borate compounds are typically ionic in character containing a boron oxoanion, such as the metaborate anion [B(OH)4]−, in combination with cationic species, such as sodium, Na+. For example sodium metaborate is Na+[B(OH)4]−, also written as Na2B2O4.4H2O or as the resolved oxide formula, Na2O.B2O3.4H2O. Table 1 lists a number of well-known borate anions, including the structural formula and the resolved oxide formula for the corresponding compounds formed in combination with monovalent cations (represented by M+).
TABLE 1Structural FormulaResolvedBoronCommonin Combination withOxideOxoanionNameCation M+Formula[B(OH)4]−metaborateM[B(OH)4]M2O.B2O3.4H2O[B4O5(OH)4]2−tetraborateM2[B4O5(OH)4]M2O.2B2O3.4H2O[B5O6(OH)4]−pentaborateM[B5O6(OH)4]M2O.5B2O3.4H2O[B3O3(OH)4]−triborateM[B3O3(OH)4]M2O.3B2O3.4H2O[B3O3(OH)5]2−triborateM2[B3O3(OH)5]2M2O.3B2O3.5H2O[B6O7(OH)6]2−hexaborateM2[B6O7(OH)6]M2O.3B2O3.3H2O
Structurally, each of these borate oxoanions has a different form. For example the metaborate anion has a tetrahedral form, while the tetraborate anion is a bridged eight-member B—O ring, the triborate anion is a six-member B—O ring (referred to as a boroxyl ring), the pentaborate anion consists of two six-member B—O rings sharing a common boron atom and the hexaborate anion consists of three B—O rings that share three boron atoms and one oxygen atom. Structural representations of several of these borate oxoanions are shown in Formulas I-V, below. 
In the formation of natural minerals and synthetic borate compounds, these boron oxoanions are either isolated (finite), i.e. connected to adjacent boron oxoanions only by hydrogen bonds and not by oxygen bridges, or alternatively they are directly interconnected through boron-oxygen bonding into infinite chains, sheets or 3-dimensional framework structures. Borate minerals and synthetic compounds made up of isolated boron oxoanions containing from one to six boron atoms are quite common. Similarly, borate minerals and compounds containing infinite chains, sheets and 3-dimensional framework structures made up of repeating boron oxoanions (fundamental building blocks) having more than six boron atoms, such as the mineral preobrazhenskite, are also well known (Burns, P. C. et al., “Borate Minerals. I. Polyhedral Clusters and Fundamental Building Blocks”, in The Canadian Mineralogist, 33, 1995, p. 1132-1133 and Grice, J. D. et al., “Borate Minerals. II. A Hierarchy of Structures Based on the Borate Fundamental Building Block”, in The Canadian Mineralogist, 37, 1999, p. 731-762). However, isolated boron oxoanions having more than six boron atoms are rare and none have been reported with nine boron atoms.
Several amine borate compounds containing the guanidinium cation have been reported in the literature. Rosenheim and Leyser prepared guanidinium tetraborate (which they referred to as “biborate”) by boiling guanidinium carbonate with the corresponding quantity of boric acid until no more carbon dioxide escaped, Z. Anorg. U. Allg. Chem., 119, 1921, p. 1-38. They also prepared guanidinium tetraborate by precipitation in cold water by the treatment of concentrated borax solution with guanidinium chloride.
Heller described the preparation of a guanidinium pentaborate compound, formulated as [C(NH2)3][B5O6(OH)4], by the reaction of boric acid with guanidinium carbonate in boiling water, J. Inorg. Nucl. Chem., Vol. 30, 1968, p. 2743-2754. Three crystalline guanidinium borate compounds containing free urea were prepared by hydrolysis of trialkoxyboron compounds with guanidine in ethanol and hydrocarbon solvent. These included [C(NH2)3]2[B3O3(OH)5].(NH2)2C═O, [C(NH2)3]3[B4O5(OH)4].2(NH2)2C═O, and [C(NH2)3]4[B5O6(OH)7].(NH2)2C═O.
G. H. Bowden, in “Supplement to Mellor's Comprehensive Treatise on Inorganic and Theoretical Chemistry”, Vol. V, Boron, Part A: Boron-Oxygen Compounds, described two guanidinium borate compounds. The first is “guanidinium diborate” (i.e., tetraborate), [(NH2)2═C═NH2]2O.2B2O3.4H2O, which is formed by reacting theoretical quantities of guanidinium carbonate and boric acid in a minimum of water at 90° C. Bowden also suggested that this compound may be produced from guanidinium chloride and borax. The second compound, guanidinium pentaborate hexahydrate, [(NH2)2═C═NH2]2O.5B2O3.6H2O, was formed from guanidinium carbonate and the theoretical quantity of boric acid, in the same way as the diborate and can also be prepared from a mixture of borax and boric acid by treatment with guanidinium chloride. Bowden lists the pentaborate compound among several borate compounds investigated for possible use in citrus fruit preservation.
Ono Hiroshi described a method of producing boron nitride by heating a guanidine borate compound in inert or reductive atmosphere at 600° C. or higher in, “Manufacturing Method of Boron Nitride”, JP 95,102,965 (JP 63,011,505), 1995, Mitsu Toatsu Chemicals, Inc. The guanidine borate compounds proposed for use are of the composition x[C(NH2)3]2O.yB2O3.zH2O (where x and y can be 1-5 and z can be 0-9), which are prepared by the reaction of free guanidine with boric acid or boric oxide in water in the appropriate ratios. Free guanidine was prepared by passing a solution of guanidine carbonate through a column of strongly basic ionic exchange resin to remove the carbonate radical. Hiroshi further suggests that guanidine can be replaced by substituted guanidine (R1R2NC(═NH)NR3R4, where R1-4═H, alkyl, aryl, cyanoalkyl, or hydroxyalkyl, and R1 may be an amino group).
Tetsuo Yoshiyama described the preparation of guanidinium borate compounds by reaction of guanidine and a variety of substituted guanidine compounds (R1R2NC(═NH)NR3R4, where R1-4═H, alkyl, aryl, or hydroxyalkyl, and R1 may be an amino group) with alkoxyboron compounds in organic solvent and water (see “Manufacturing Method of Borate of Guanidine Compound”, JP 94,041,048 (JP 06,041,048), 1994, Mitsu Toatsu Chemicals, Inc. The product obtained using unsubstituted guanidine was a non-crystalline, alkaline, solid powder having the chemical composition 10.8% C, 7.0% H, 32.8% N, and 9.3% B. This composition is similar to [C(NH2)3]2[B3O3(OH)5].(NH2)2C═O produced by Heller.
Applicant has unexpectedly found a family of borate compounds containing an isolated nonaborate oxoanion which is the first reported example of an isolated nonaborate anion in a hydrated borate (the term “hydrated borate” refers to borate compounds that possess B—OH groups, which may or may not also contain free water of crystallization.)